1. Field of the Invention
The present invention relates to a transmission circuit used for communication devices such as mobile phones and wireless LAN devices, and particularly to a transmission circuit, which operates with low distortion and high efficiency, and a communication device using the transmission circuit.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to, even when operating over a wide bandwidth, secure precision of a transmission signal while operating with low power consumption. For such a communication device, a transmission circuit, which is capable of outputting a transmission signal with a high precision regardless of a bandwidth and operating with high efficiency, is used. Hereinafter, conventional transmission circuits will be described.
One of the conventional transmission circuits is, for example, a transmission circuit which uses a modulation method such as a quadrature modulation method to generate a transmission signal (hereinafter, referred to as a quadrature modulation circuit). Since the quadrature modulation circuit is well known, the description thereof will be omitted. A conventional transmission circuit, which is smaller in size and operates more efficiently than the quadrature modulation circuit, is, e.g., a transmission circuit 500 shown in FIG. 18. FIG. 18 is a block diagram showing an exemplary configuration of the conventional transmission circuit 500. In FIG. 18, the conventional transmission circuit 500 comprises a signal generation section 501, output terminal 502, amplitude amplifying section 503, amplitude modulation section 504 and a power supply terminal 505.
In the conventional transmission circuit 500, the signal generation section 501 outputs an amplitude signal and angle modulation signal. The amplitude signal is inputted to the amplitude amplifying section 503. The amplitude amplifying section 503 supplies, to the amplitude modulation section 504, a voltage corresponding to a magnitude of the inputted amplitude signal. To the amplitude amplifying section 503, a DC voltage is supplied from the power supply terminal 505. Typically, the amplitude amplifying section 503 supplies, to the amplitude modulation section 504, a voltage proportional to the magnitude of the inputted amplitude signal.
The angle modulation signal outputted from the signal generation section 501 is inputted to the amplitude modulation section 504. The amplitude modulation section 504 amplitude-modulates the angle modulation signal by using the voltage supplied from the amplitude amplifying section 503 (in this example, a collector voltage Vc), and outputs a resultant signal as a modulation signal having been angle-modulated and amplitude-modulated. This modulation signal is outputted from the output terminal 502 as a transmission signal. The transmission circuit 500 which operates in the above manner is called a polar modulation circuit.
The conventional transmission circuit 500 cannot always output a transmission signal with a high precision, depending on a characteristic of the amplitude modulation section 504. Described below with reference to FIG. 19 is a characteristic of the amplitude modulation section 504. FIG. 19 shows a relationship between the collector voltage Vc supplied to the amplitude modulation section 504 and an output voltage Vo. Here, a magnitude of an input voltage (angle modulation signal) is fixed. As shown in FIG. 19, when an HBT (Heterojunction Bipolar Transistor) is used as the amplitude modulation section 504, the amplitude modulation section 504 is unable to output a voltage unless the collector voltage Vc is a particular threshold value or greater, i.e., the amplitude modulation section 504 is unable to output the output voltage Vo linearly to the inputted collector voltage Vc. Hereinafter, this characteristic of the amplitude modulation section 504 is referred to as an offset characteristic
As shown in FIG. 19, the offset characteristic changes in accordance with, e.g., a temperature of the amplitude modulation section 504 and an individual difference of the amplitude modulation section 504. Such a change is mainly caused by a characteristic of the HBT used for the amplitude modulation section 504. For example, the offset characteristic of the amplitude modulation section 504 changes when the temperature of the amplitude modulation section 504 changes from a low temperature to a room temperature, or from a room temperature to a high temperature. It is assumed in the example of FIG. 19 that the temperature of the amplitude modulation section 504 ranges from −25° C. to 120° C., and the room temperature is approximately 25° C. FIG. 19 shows an example in which a gradient of a line indicating the relationship between the collector voltage Vc and the output voltage Vo is fixed. However, there is a case where the gradient of the line changes in accordance with the temperature of the amplitude modulation section 504.
U.S. Pat. No. 6,998,919 (hereinafter, referred to as Patent Document 1) discloses a transmission circuit 600, which compensates for the offset characteristic of the amplitude modulation section 504 in accordance with the temperature of the amplitude modulation section 504. FIG. 20 is a block diagram showing an exemplary configuration of the conventional transmission circuit 600 disclosed in Patent Document 1. As shown in FIG. 20, the conventional transmission circuit 600 comprises the signal generation section 501, the output terminal 502, the amplitude amplifying section 503, the amplitude modulation section 504, the power supply terminal 505, a temperature sensor 601 and an offset compensation section 602. The temperature sensor 601 measures the temperature of the amplitude modulation section 504. The offset compensation section 602 changes, in accordance with the temperature of the amplitude modulation section 504 which has been measured by the temperature sensor 601, a magnitude of an inputted amplitude signal, thereby compensating for the offset characteristic of the amplitude modulation section 504.
However, there is a possibility that in the conventional transmission circuit 600 (see FIG. 20), the offset characteristic in relation to the temperature of the amplitude modulation section 504 is not always fixed because of, e.g., an individual difference or the like of the amplitude modulation section 504, and this causes inconsistency of an output signal from the amplitude modulation section 504. Thus, the conventional transmission circuit 600 has a problem that the offset characteristic of the amplitude modulation section 504 is not precisely compensated for, whereby distortion occurs in a transmission signal.